1. Field of the Invention
The present invention relates to a circuit module that includes a circuit board including mount components on a surface thereof, and an insulating layer covering the mount components. In particular, the present invention relates to a circuit module that allows checking of filling properties of an insulating resin layer between a circuit board and mount components.
2. Description of the Related Art
As an example of a circuit module of the related art, a circuit module described in Japanese Unexamined Patent Application Publication No. 2006-203652 is known. FIG. 6 is a cross-sectional view of this circuit module. As illustrated, a circuit module 10 includes a circuit board 12 having an inner electrode pattern composed of via holes and in-plane wiring electrodes; mount components 11, 13, 15, and 17 mounted on one principal surface of the circuit board 12; and an insulating resin layer 19 formed on the one principal surface of the circuit board 12 to cover the mount components 11, 13, 15, and 17.
The mount components 11, 13, 15, and 17 are mounted on the one principal surface of the circuit board 12 by a technique such as solder bonding or thermocompression bonding, and are connected to the inner electrode pattern of the circuit board 12. For example, the mount components 11, 15, and 17 are mounted on the circuit board 12, with connection conductors 14 interposed therebetween. In a space between the one principal surface of the circuit board 12 and bottom surfaces of the mount components 11, 15, and 17, gaps having a height of the connection conductors 14 are created in portions where no connection conductor 14 is present.
The insulating resin layer 19 is formed to entirely cover the mount components 11, 13, 15, and 17 including the gaps described above. The insulating resin layer 19 not only reinforces the mechanical connection between the circuit board 12 and the mount components 11, 13, 15, and 17, but also prevents the mount components 11, 13, 15, and 17 from being damaged by external shock.
To lower the profile of the circuit module 10, the connection conductors 14 are often designed to be low in height. This means that the height of the gaps between the circuit board 12 and the mount components 11, 15, and 17 is very small (about several tens of micrometers).
Therefore, when the insulating resin layer 19 is formed on the circuit board 12, the gaps may not be fully filled with the insulating resin layer 19 and air bubbles may remain. In this case, when the circuit module 10 is solder-bonded to a printed wiring board or the like by reflowing which involves heating, the resulting thermal expansion of the remaining air bubbles may cause cracks in the insulating resin layer 19 or the mount components 11, 15, and 17.
Moreover, if moisture remains in the air bubbles or gradually permeates through the insulating resin layer 19 and accumulates in the air bubbles, outer electrodes of the mount components 11, 15, and 17 may be corroded.
The presence of the air bubbles may weaken the mechanical connection between the circuit board 12 and the mount components 11, 15, and 17.